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bubbles in Minwax Polycrylic finish

systembuilder's picture

I've used Minwax water-based Polycrylic on a few projects now. I think it gives a pretty good finish without a lot of fuss. However, it seems I always fight with bubbles in the cured finish. I typically use a synthetic brush to flow it on. I'm now experimenting with one of those paint pads (made by Shur-line). Is this a common complaint, or is my technique just bad? I try not to over brush. The pad (after just one try) seems a better way to achieve a bubble-freee finish.

JohnWW's picture

(post #111689, reply #1 of 13)

A common cause for bubbles is dipping the brush and then wiping it off on the edge of the can. The solution is to dip in just enough to load the brush properly and then go straight to the workpiece without striking off the excess. Of course being careful not to stir in bubbles helps too.

John W.

John White Shop Manager for FWW Magazine, 1998 to 2007

systembuilder's picture

(post #111689, reply #2 of 13)

John,


Thx, I'll be careful about that.


When I do get bubbles, I sand the entire surface and then put on a fresh coat. I have been unsuccessful in sanding bubbles locally and then applying another coat just in that localized area. The new coat never blends seamlessly into the prior coat. Is this normal?


Thx for your response.


Dan

JohnWW's picture

(post #111689, reply #3 of 13)

Because, with that kind of finish, the new coat doesn't melt into the coat below you will almost always have a visible difference when you try to do a spot repair. Some finishes, like shellac will melt into the previous layer and they will blend in invisibly.

John W.

John White Shop Manager for FWW Magazine, 1998 to 2007

Kenbry's picture

(post #111689, reply #4 of 13)

When I get bubbles in the poly finish I use a clean, dry foam brush to remove them. Simply and gently drag the tip of the foam brush down the entire length of the project. I also like to use the sure-line 4" foam rollers when applying polyurethane. The two foam applicators make a good team.


Hope this helps.


Bryken 

MrBill's picture

(post #111689, reply #5 of 13)

sb,


 I just finished a shadow box for my Wife last weekend and had no problem, and the can was probably 2 years old :(  I use a synthetic brush and no particularly special technique. I did three coats on Saturday and just barely had to scuff them between coats. No bubbles that I can remember.


 I agree with the others about not "striking off" the brush. My Dad taught me many years ago to "slap" the brush against the edge of the can if you get too much paint on it, never to wipe it across the edge of the can. This has always worked well for me. Also, stir the poly slowly and never shake the can.


Bill Koustenis


Advanced Automotive Machine


Waldorf Md

Bill Koustenis

Advanced Automotive Machine

Waldorf Md

blewcrowe's picture

(post #111689, reply #6 of 13)

Sometimes I think these formulations are a bit thick, and that exacerbates the bubbling problem. I suppose, if careful, you might thin it by 10%.


But like one fellow's advice, if you keep a similar but slightly drier brush handy, you can use it to make a swipe and get rid of the bubbles.


Another technique, if you've not tried it, is called "tipping off". And that is where you go back with the same brush, but held close to 90 degrees perpendicular to the surface, and ever so lightly, drag. This tends to remove bubbles, too.


All that said, I've had better outcomes with the Zar than I have with the Minwax.

 

Denny

10fingers's picture

(post #111689, reply #7 of 13)

Hello,


My instructor in crafts class showed us how to use a propane torch to evaporate the bubbles in a bartop finish.  It might work for you with water based poly.  Nothing ever caught fire.  We just held the torch about 12" over the top of the item finished and slowly moved the torch.


10fingers

HowardAcheson's picture

(post #111689, reply #8 of 13)

That will work with a pour-on epoxy finish but would be no value with either an oil based or waterborne finish.

On the issue of bubbles in the finish, they come from either shaking the can of finish or during the brushing on of the finish. Almost always it's the latter. Moving the brush too fast, vigorous back and forth "brushing" and failure to tip off are the main causes. Also, applying the finish in colder temperatures can also cause bubbles.

Apply the finish with long strokes. Work on a small (2'x2') area. Finish by getting most of the finish out of the brush and using long, slow strokes just touch the brush tips to the finish. This tipping off should break up an bubbles and allow the finish to flow out smoothly.

Howie.........
Howie.........
BruceS's picture

(post #111689, reply #9 of 13)

Maybe there is an "underlaying" cause.  What is under the finish?  Or are you going to raw lumber?


An oil based stain, or wipe down with mineral spirits may still be out gassing.


Work Safe,  Count to 10 when your done for the day !!


Bruce S. 


 

Work Safe,  Count to 10 when your done for the day !!

Bruce S. 

 

systembuilder's picture

(post #111689, reply #10 of 13)

Bruce,


The wood had alcohol based aniline dyes applied first, then BLO, then multiple coats of shellac. Finally the water based finish coats.


The "tipping off" suggestions have cured my issues. I will also try thinning by 10% with distilled water next time (which is very soon).


Thx all for the help.


Dan

HowardAcheson's picture

(post #111689, reply #12 of 13)

>> then multiple coats of shellac.

What type of shellac did you use? Was it a dewaxed shellac. You may already know that a waterborne finish should not be applied over a shellac that contains its natural wax.

Finally, why did you apply multiple coats of shellac?

Howie.........
Howie.........
systembuilder's picture

(post #111689, reply #13 of 13)

Howie,


I used Behlen's Super Blonde shellac flakes, which the label states are 99% wax free. I mixed 2 oz. (weight) of flakes with 16 oz. (volume) of DNA. This gave me a 1 pound cut. I applied with a foam brush. The thickness of the layers was thicker than I expected for a 1 pound cut, given my past experience though maybe I am just grasping for clues.


You asked why I used multiple coats of shellac, since it's only a barrier coat between the BLO and the water based finish. Your response to my answer will help my understanding greatly. I used multiple coats because I had some lap marks with the first (and second) coats of shellac. I was sanding between coats and hoping the successive layers would burn through those beneath so that I could get a very flat surface before switching to water based. I though this was essential because 1) I was afraid the lap marks in the shellac would show through the water based, and 2) I was going to rub out and flatten the final water based layers and was afraid I would cut through the water based layers and expose the high areas of the shellac below it.


Now it so happens that I sanded the heck out of the table top with 220, multiple times and vaccuming after every sanding. I got rid of all the lap marks and probably came very close to getting down to bare wood. I just put on a coat of 1 pound shellac and it looks very good, though not perfect. I now have to decide to sand and apply a second coat or go directly to water based, so your comments to this post will be very enlightening to me.


Thx and regards,


Dan

Jigs-n-fixtures's picture

(post #111689, reply #11 of 13)

If you use one of the pads, vacuum it first to get any loose bristles off.